[NTG-context] referencing position on page in index

Hi all, one of my favorite grammars contains rather elaborated indices: to a given entry they not only list the page numbers, but also the position on the respective page. To this end any page is subdivided into four regions to which the entries may refer separately. For instance, these entries: Aspekte beim Verbum 509\high{2} Assibilation 154\high{2·4} would refer to the second quarter of page 509 and to the second and fourth quarter of page 154 respectively. Is there a mechanism to achieve that or could the context’s referencing be extended in that way? Some page scans from the indices: http://bayimg.com/pAAhkAaDn http://bayimg.com/aAAiJAaDn Thanks for your time, Philipp